Heterostructured Visible‐Light‐Active Photocatalyst of Chromia‐Nanoparticle‐Layered Titanate

An efficient visible‐light active photocatalyst of porous CrO_x–Ti_1.83O₄ nanohybrid with a 1:1 type ordered heterostructure is synthesized through a hybridization between a chromia cluster and exfoliated titanate nanosheets. The present nanohybrids are found to have a large surface area (ca. 250–310 m² g^–1) and an intense absorption of visible light, ascribable, respectively, to the formation of a porous structure and the hybridization of titanate with narrow‐bandgap chromium oxide. After the calcination at 400 °C, the nanohybrid shows an enhanced photocatalytic activity to effectively decompose organic compounds under the irradiation of visible light (λ > 420 nm). The present study highlights the exfoliation–restacking route as a very powerful way to develop efficient visible‐light‐harvesting photocatalysts with excellent thermal stability.     

g‐C3N4 Loading Black Phosphorus Quantum Dot for Efficient and Stable Photocatalytic H2 Generation under Visible Light

Black phosphorus (BP) is an interesting two‐dimensional material with low‐cost and abundant metal‐free properties and is used as one cocatalyst for photocatalytic H₂ production. However, the BP quantum dot (BPQD) is not studied. Herein, for the first time, BPQD is introduced as a hole‐migration cocatalyst of layered g‐C₃N₄ for visible‐light‐driven photocatalytic hydrogen generation. A high‐vacuum stirring method is developed for BPQD loading without the dissociation of BP. The layered BPQD is coupled on the layered g‐C₃N₄ surface to form a heterojunction structure. The 7% BPQD–C₃N₄ samples show similar time‐resolved photoluminescence curves as 0.5% Pt–C₃N₄. The optimum hydrogen rates of the modified sample (7% BPQD–C₃N₄) are 190, 133, 90, and 10.4 µmol h^?1 under simulated sunlight, LED‐405, LED‐420, and LED‐550 nm irradiation, respectively, which are 3.5, 3.6, and 3 times larger than that of the pristine g‐C₃N₄. Such low‐cost layered system not only optimizes the optical, electrical, and texture properties of the hybrid materials for photocatalytic water splitting to generate hydrogen but also provides ideas for designing novel or easily oxidized candidates by incorporating different available materials with given carriers.     

Localized Surface Plasmon Resonance Enhanced Photocatalytic Hydrogen Evolution via Pt@Au NRs/C3N4 Nanotubes under Visible‐Light Irradiation

Au nanorods (NRs) decorated carbon nitride nanotubes (Au NRs/CNNTs) photocatalysts have been designed and prepared by impregnation–annealing approach. Localized surface plasmon resonance (LSPR) peaks of Au NRs can be adjusted by changing the aspect ratios, and the light absorption range of Au NRs/CNNTs is extended to longer wavelength even near‐infrared light. Optimal composition of Pt@Au NR₇₆₉/CNNT₆₅₀ has been achieved by adjusting the LSPR peaks of Au NRs and further depositing Pt nanoparticles (NPs), and the photocatalytic H₂ evolution rate is 207.0 µmol h^?1 (20 mg catalyst). Preliminary LSPR enhancement photocatalytic mechanism is suggested. On one hand, LSPR of Au NRs is beneficial for visible‐light utilization. On the other hand, Pt NPs and Au NRs have a synergetic enhancement effect on photocatalytic H₂ evolution of CNNTs, in which the local electromagnetic field can improve the photogenerated carrier separation and direct electron transfer increases the hot electron concentration while Au NRs as the electron channel can well restrain charge recombination, finally Pt as co‐catalyst can boost H⁺ reduction rate. This work provides a new way to develop efficient photocatalysts for splitting water, which can simultaneously extend light absorption range and facilitate carrier generation, transportation and reduce carrier recombination.     

Improved Hydrogen Production of Au–Pt–CdS Hetero‐Nanostructures by Efficient Plasmon‐Induced Multipathway Electron Transfer

The design of new functional materials with excellent hydrogen production activity under visible‐light irradiation has critical significance for solving the energy crisis. A well‐controlled synthesis strategy is developed to prepare an Au–Pt–CdS hetero‐nanostructure, in which each component of Au, Pt, and CdS has direct contact with the other two materials; Pt is on the tips and a CdS layer along the sides of an Au nanotriangle (NT), which exhibits excellent photocatalytic activity for hydrogen production under light irradiation (λ > 420 nm). The sequential growth and surfactant‐dependent deposition produce the three‐component Au–Pt–CdS hybrids with the Au NT acting as core while Pt and CdS serve as a co‐shell. Due to the presence of the Au NT cores, the Au–Pt–CdS nanostructures possess highly enhanced light‐harvesting and strong local‐electric‐field enhancement. Moreover, the intimate and multi‐interface contact generates multiple electron‐transfer pathways (Au to CdS, CdS to Pt and Au to Pt) which guide photoexcited electrons to the co‐catalyst Pt for an efficient hydrogen reduction reaction. By evaluating the hydrogen production rate when aqueous Na₂SO₃–Na₂S solution is used as sacrificial agent, the Au–Pt–CdS hybrid exhibits excellent photocatalytic activity that is about 2.5 and 1.4 times larger than those of CdS/Pt and Au@CdS/Pt, respectively.     

Ice‐Assisted Synthesis of Black Phosphorus Nanosheets as a Metal‐Free Photocatalyst: 2D/2D Heterostructure for Broadband H2 Evolution

A 2D/2D heterojunction of black phosphorous (BP)/graphitic carbon nitride (g‐C₃N₄) is designed and synthesized for photocatalytic H₂ evolution. The ice‐assisted exfoliation method developed herein for preparing BP nanosheets from bulk BP, leads to high yield of few‐layer BP nanosheets (≈6 layers on average) with large lateral size at reduced duration and power for liquid exfoliation. The combination of BP with g‐C₃N₄ protects BP from oxidation and contributes to enhanced activity both under λ > 420 nm and λ > 475 nm light irradiation and to long‐term stability. The H₂ production rate of BP/g‐C₃N₄ (384.17 µmol g^?1 h^?1) is comparable to, and even surpasses that of the previously reported, precious metal‐loaded photocatalyst under λ > 420 nm light. The efficient charge transfer between BP and g‐C₃N₄ (likely due to formed N? P bonds) and broadened photon absorption (supported both experimentally and theoretically) contribute to the excellent photocatalytic performance. The possible mechanisms of H₂ evolution under various forms of light irradiation is unveiled. This work presents a novel, facile method to prepare 2D nanomaterials and provides a successful paradigm for the design of metal‐free photocatalysts with improved charge‐carrier dynamics for renewable energy conversion.     

Surface Ligands Stabilized Lead Halide Perovskite Quantum Dot Photocatalyst for Visible Light‐Driven Hydrogen Generation

Solar hydrogen conversion represents a clean and economic approach to addressing global energy and environmental issues, for which efficient photocatalysts are heavily pursued. Lead halide perovskites are promising candidates for efficient phtocatalysts in solar hydrogen generation due to their attractive properties in light absorption, photogenerated charge transportation, and utilization. However, photocatalytic applications of lead halide perovskites are limited owing to their poor stability in the presence of water or other polar solvent environment. This work presents the rational control of surface ligands in achieving a good balance between stability and photocatalytic activity of CsPbBr₃ quantum dots (QDs). Detailed studies reveal that the deliberate surface ligands engineering is crucial for maximizing the photocatalytic activity of CsPbBr₃ QDs while maintaining good QD stability. A certain amount of surface ligands protect the CsPbBr₃ QDs from decomposition in moisture during the photocatalytic reaction while still enabling efficient charge transfer for photocatalytic reactions on the surface of QDs. The well‐controlled CsPbBr₃ photocatalyst shows efficient visible light‐driven H₂ generation with outstanding stability (≥160 h).     

Highly Efficient Quantum‐Dot‐Sensitized Solar Cell Based on Co‐Sensitization of CdS/CdSe

Cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dots (QDs) are sequentially assembled onto a nanocrystalline TiO₂ film to prepare a CdS/CdSe co‐sensitized photoelectrode for QD‐sensitized solar cell application. The results show that CdS and CdSe QDs have a complementary effect in the light harvest and the performance of a QDs co‐sensitized solar cell is strongly dependent on the order of CdS and CdSe respected to the TiO₂. In the cascade structure of TiO₂/CdS/CdSe electrode, the re‐organization of energy levels between CdS and CdSe forms a stepwise structure of band‐edge levels which is advantageous to the electron injection and hole‐recovery of CdS and CdSe QDs. An energy conversion efficiency of 4.22% is achieved using a TiO₂/CdS/CdSe/ZnS electrode, under the illumination of one sun (AM1.5,100 mW cm^?2). This efficiency is relatively higher than other QD‐sensitized solar cells previously reported in the literature.     

Size Effects of Platinum Nanoparticles in the Photocatalytic Hydrogen Production Over 3D Mesoporous Networks of CdS and Pt Nanojunctions

Catalysts for the photogeneration of hydrogen from water are key for realizing solar energy conversion. Despite tremendous efforts, developing hydrogen evolution catalysts with high activity and long‐term stability remains a daunting challenge. Herein, the design and fabrication of mesoporous Pt‐decorated CdS nanocrystal assemblies (NCAs) are reported, and their excellent performance for the photocatalytic hydrogen production is demonstrated. These materials comprise varying particle size of Pt (ranging from 1.8 to 3.3 nm) and exhibit 3D nanoscale pore structure within the assembled network. Photocatalytic measurements coupled with UV–vis/NIR optical absorption, photoluminescence, and electrochemical impedance spectroscopy studies suggest that the performance enhancement of these catalytic systems arises from the efficient hole transport at the CdS/electrolyte interface and interparticle Pt/CdS electron‐transfer process as a result of the deposition of Pt. It is found that the Pt‐CdS NCAs catalyst at 5 wt% Pt loading content exerts a 1.2 mmol h^?1 H₂‐evolution rate under visible‐light irradiation (λ ≥ 420 nm) with an apparent quantum yield of over 70% at wavelength λ = 420 nm in alkaline solution (5 m NaOH), using ethanol (10% v/v) as sacrificial agent. This activity far exceeds those of the single CdS and binary noble metal/CdS systems, demonstrating the potential for practical photocatalytic hydrogen production.     

Engineering the Photocatalytic Behaviors of g/C3N4‐Based Metal‐Free Materials for Degradation of a Representative Antibiotic

Graphitic carbon nitride (g/C₃N₄) is of promise as a highly efficient metal‐free photocatalyst, yet engineering the photocatalytic behaviours for efficiently and selectively degrading complicated molecules is still challenging. Herein, the photocatalytic behaviors of g/C₃N₄ are modified by tuning the energy band, optimizing the charge extraction, and decorating the cocatalyst. The combination shows a synergistic effect for boosting the photocatalytic degradation of a representative antibiotic, lincomycin, both in the degradation rate and the degree of decomposition. In comparison with the intrinsic g/C₃N₄, the structurally optimized photocatalyst shows a tenfold enhancement in degradation rate. Interestingly, various methods and experiments demonstrate the specific catalytic mechanisms for the multiple systems of g/C₃N₄‐based photocatalysts. In the degradation, the active species, including ·O₂^?, ·OH, and h⁺, have different contributions in the different photocatalysts. The intermediate, H₂O₂, plays an important role in the photocatalytic process, and the detailed functions and originations are clarified for the first time.     

Large‐Size,Porous, Ultrathin NiCoP Nanosheets for Efficient Electro/Photocatalytic Water Splitting

Porous ultrathin 2D catalysts are attracting great attention in the field of electro/photocatalytic hydrogen evolution reaction (HER) and overall water splitting. Herein, a universal pH‐controlled wet‐chemical strategy is reported followed by thermal and phosphorization treatment to prepare large‐size, porous and ultrathin bimetallic phosphide (NiCoP) nanosheets, in which graphene oxide is adopted as a template to determine the size of products. The thickness of the resultant NiCoP nanosheets ranges from 3.5 to 12.8 nm via delicately adjusting pH from 7.8 to 8.5. The thickness‐dependent electrocatalytic performance is evidenced experimentally and explained by computational studies. The prepared large‐size ultrathin NiCoP nanosheets show excellent bifunctional electrocatalytic activity for overall water splitting, with low overpotentials of 34.3 mV for HER and 245.0 mV for oxygen evolution reaction, respectively, at 10 mA cm^?2. Furthermore, the NiCoP nanosheets exhibit superior photocatalytic HER performance, achieving a high HER rate of 238.2 mmol h^?1 g^?1 in combination with commonly used photocatalyst CdS, which is far superior to that of Pt/CdS (81.7 mmol h^?1 g^?1). All these results demonstrate large‐size porous ultrathin NiCoP nanosheets as an efficient and multifunctional electro/photocatalyst for water splitting.     

A Cocatalytic Electron‐Transfer Cascade Site‐Selectively Placed on TiO2 Nanotubes Yields Enhanced Photocatalytic H2 Evolution

Separation and transfer of photogenerated charge carriers are key elements in designing photocatalysts. TiO₂ in numerous geometries has been for many years the most studied photocatalyst. To overcome kinetic limitations and achieve swift charge transfer, TiO₂ has been widely investigated with cocatalysts that are commonly randomly placed nanoparticles on a TiO₂ surface. The poor control over cocatalyst placement in powder technology approaches can drastically hamper the photocatalytic efficiencies. Here in contrast it is shown that the site‐selective placement of suitable charge‐separation and charge‐transfer cocatalysts on a defined TiO₂ nanotube morphology can provide an enhancement of the photocatalytic reactivity. A TiO₂–WO₃–Au electron‐transfer cascade photocatalyst is designed with nanoscale precision for H₂ production on TiO₂ nanotube arrays. Key aspects in the construction are the placement of the WO₃/Au element at the nanotube top by site‐selective deposition and self‐ordered thermal dewetting of Au. In the ideal configuration, WO₃ acts as a buffer layer for TiO₂ conduction band electrons, allowing for their efficient transfer to the Au nanoparticles and then to a suitable environment for H₂ generation, while TiO₂ holes due to intrinsic upward band bending in the nanotube walls and short diffusion length undergo a facilitated transfer to the electrolyte where oxidation of hole‐scavenger molecules takes place. These photocatalytic structures can achieve H₂ generation rates significantly higher than any individual cocatalyst–TiO₂ combination, including a classic noble metal–TiO₂ configuration.     

Graphene‐Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities

“Graphitic” (g)‐C₃N₄ with a layered structure has the potential of forming graphene‐like nanosheets with unusual physicochemical properties due to weak van der Waals forces between layers. Herein is shown that g‐C₃N₄ nanosheets with a thickness of around 2 nm can be easily obtained by a simple top‐down strategy, namely, thermal oxidation etching of bulk g‐C₃N₄ in air. Compared to the bulk g‐C₃N₄, the highly anisotropic 2D‐nanosheets possess a high specific surface area of 306 m² g^?1, a larger bandgap (by 0.2 eV), improved electron transport ability along the in‐plane direction, and increased lifetime of photoexcited charge carriers because of the quantum confinement effect. As a consequence, the photocatalytic activities of g‐C₃N₄ nanosheets have been remarkably improved in terms of ?OH radical generation and photocatalytic hydrogen evolution.     

Synthesis of Leaf‐Vein‐Like g‐C3N4 with Tunable Band Structures and Charge Transfer Properties for Selective Photocatalytic H2O2 Evolution

Photocatalytic H₂O₂ evolution through two‐electron oxygen reduction has attracted wide attention as an environmentally friendly strategy compared with the traditional anthraquinone or electrocatalytic method. Herein, a biomimetic leaf‐vein‐like g‐C₃N₄ as an efficient photocatalyst for H₂O₂ evolution is reported, which owns tenable band structure, optimized charge transfer, and selective two‐electron O₂ reduction. The mechanism for the regulation of band structure and charge transfer is well studied by combining experiments and theoretical calculations. The H₂O₂ yield of CN4 (287 µmol h^?1) is about 3.3 times higher than that of pristine CN (87 µmol h^?1), and the apparent quantum yield for H₂O₂ evolution over CN4 reaches 27.8% at 420 nm, which is much higher than that for many other current photocatalysts. This work not only provides a novel strategy for the design of photocatalyst with excellent H₂O₂ evolution efficiency, but also promotes deep understanding for the role of defect and doping sites on photocatalytic activity.     

Thin‐Layered Photocatalysts

Semiconductor photocatalysis, a green and sustainable technology, is of great significance for solving environmental pollution and energy shortages. However, the common problems of inefficient light harvesting, rapid recombination of electron–hole pairs, and low surface reactive reaction sites for photocatalysts urgently need to be solved. In this regard, thin‐layered photocatalysts are considered to be one of the most promising candidates for addressing these issues, due to their unique surface and electronic properties. In this review, the various strategies for constructing thin‐layered photocatalysts are summarized, and emphasis is given to approaches for optimizing the photocatalytic performance of the thin‐layered materials, which can be classified into surface engineering and junction construction. In addition, the photocatalytic applications of thin‐layered materials, i.e., water splitting, CO₂ reduction, nitrogen fixation, and molecule oxygen activation, are summarized. Finally, based on current achievements in thin‐layered photocatalysts, their future development and challenges are discussed.     

Spatially Separated CdS Shells Exposed with Reduction Surfaces for Enhancing Photocatalytic Hydrogen Evolution

To the photocatalytic H₂ evolution, the exposure of a reduction surface over a catalyst plays an important role for the reduction of hydrogen protons. Here, this study demonstrates the design of a noble‐metal‐free spatially separated photocatalytic system exposed with reduction surfaces (MnO_x @CdS/CoP) for highly solar‐light‐driven H₂ evolution activity. CoP and MnO_x nanoparticles are employed as the electron and hole collectors, which are selectively anchored on the outer and inner surface of CdS shells, respectively. Under solar light irradiation, the photogenerated holes and electrons can directionally move to the MnO_x and CoP, respectively, leading to the exposure of a reduction surface. As a result, the H₂ evolution increases from 32.0 to 238.4 µmol h^?1, which is even higher than the activity of platinum‐loaded photocatalyst (MnO_x @CdS/Pt). Compared to the pure CdS with serious photocorrosion, the MnO_x @CdS/CoP maintains a changeless activity for the H₂ evolution and rhodamine B degradation, even after four cycles. The research provides a new strategy for the preparation of spatially separated photocatalysts with a selective reduction surface.     

Synthesis of Nanohole‐Structured Single‐Crystalline Platinum Nanosheets Using Surfactant‐Liquid‐Crystals and their Electrochemical Characterization

Nanohole‐structured single‐crystalline Pt nanosheets have been synthesized by the borohydride reduction of Na₂PtCl₆ confined to the lyotropic liquid crystals (LLCs) of polyoxyethylene (20) sorbitan monooleate (Tween 80) with or without nonaethylene‐glycol (C₁₂EO₉). The Pt nanosheets of around 4–10 nm in central thickness and up to 500 nm or above in diameter have a number of hexagonal‐shaped nanoholes ∼1.8 nm wide. High‐resolution electron microscope images of the nanosheets showed atomic fringes with a spacing of 0.22 nm indicating that the nanosheets are crystallographically continuous through the nanoholed and non‐holed areas. The inner‐angle distributions for the hexagonal nanoholes indicate that the six sides of the nanoholes are walled with each two Pt (111), Pt (1 1) and Pt (010) planes. The formation mechanism of nanoholed Pt nanosheets is discussed on the basis of structural and compositional data for the resulting solids and their precursory LLCs, with the aid of similar nanohole growth observed for a Tween 80 free but oleic acid‐incorporated system. It is also demonstrated that the nanoholed Pt nanostructures loaded on carbon exhibit fairly high electrocatalytic activity for oxygen reduction reaction and a high performance as a cathode material for polymer‐electrolyte fuel cells, along with their extremely high thermostability revealed through the effect of electron‐irradiation.     

Delocalized Electron Accumulation at Nanorod Tips: Origin of Efficient H2 Generation

Photocatalytic hydrogen (H₂) evolution requires efficient electron transfer to catalytically active sites in competition with charge recombination. Thus, controlling charge‐carrier dynamics in the photocatalytic H₂ evolution process is essential for optimized photocatalyst nanostructures. Here, the efficient delocalization of electrons is demonstrated in a heterostructure consisting of optimized MoS₂ tips and CdS nanorods (M‐t‐CdS Nrs) synthesized by amine‐assisted oriented attachment. The heterostructure achieves photocatalytic H₂ activity of 8.44 mmol h^?1 g^?1 with excellent long‐term durability (>23 h) without additional passivation under simulated solar light (AM 1.5, 100 mW cm^?2). This activity is nearly two orders of magnitude higher than that of pure CdS Nrs. The impressive photocatalytic H₂ activity of M‐t‐CdS Nrs reflects favorable charge‐carrier dynamics, as determined by steady‐state PL and time‐correlated single photon counting correlation analysis at low temperature. The MoS₂ cocatalysts precisely located at the end of the CdS Nrs exhibit ultrafast charge transfer and slow charge recombination via spatially localized deeper energy states, resulting in a highly efficient H₂ evolution reaction in lactic acid containing an electrolyte.     

Efficient TiO2 Photocatalysts from Surface Hybridization of TiO2 Particles with Graphite‐like Carbon

Surface hybridization of TiO₂ with graphite‐like carbon layers of a few molecular layers thickness yields efficient photocatalysts. Photoelectrochemical measurements confirm an electronic interaction between TiO₂ and the graphite‐like carbon. A TiO₂ photocatalyst with a carbon shell of three molecular layers thickness (～1 nm) shows the highest photocatalytic activity which is about two times higher than that of Degussa P25 TiO₂ under UV light irradiation. The mechanism of the enhanced photocatalytic activity under UV irradiation is based on the high migration efficiency of photoinduced electrons at the graphite‐like carbon/TiO₂ interface, which is due to the electronic interaction between both materials. In addition, a high activity under visible light irradiation is observed after graphite‐like carbon hybridization. TiO₂'s response is extended into the visible range of the solar spectrum due to the electronic coupling of π states of the graphite‐like carbon and conduction band states of TiO₂.     

Unique PCoN Surface Bonding States Constructed on g‐C3N4 Nanosheets for Drastically Enhanced Photocatalytic Activity of H2 Evolution

Developing high‐efficiency and low‐cost photocatalysts by avoiding expensive noble metals, yet remarkably improving H₂ evolution performance, is a great challenge. Noble‐metal‐free catalysts containing Co(Fe)?N?C moieties have been widely reported in recent years for electrochemical oxygen reduction reaction and have also gained noticeable interest for organic transformation. However, to date, no prior studies are available in the literature about the activity of N‐coordinated metal centers for photocatalytic H₂ evolution. Herein, a new photocatalyst containing g‐C₃N₄ decorated with CoP nanodots constructed from low‐cost precursors is reported. It is for the first time revealed that the unique P(δ^?)?Co(δ⁺)?N(δ^?) surface bonding states lead to much superior H₂ evolution activity (96.2 µmol h^?1) compared to noble metal (Pt)‐decorated g‐C₃N₄ photocatalyst (32.3 µmol h^?1). The quantum efficiency of 12.4% at 420 nm is also much higher than the record values (≈2%) of other transition metal cocatalysts‐loaded g‐C₃N₄. It is believed that this work marks an important step toward developing high‐performance and low‐cost photocatalytic materials for H₂ evolution.     

In Situ Grown Pristine Cobalt Sulfide as Bifunctional Photocatalyst for Hydrogen and Oxygen Evolution

Herein, transition metal chalcogenides of pristine cobalt sulfides are rationally designed to act as robust bifunctional photocatalysts for visible‐light‐driven water splitting for the first time. Through moderate solvothermal route, cobalt sulfides are synthesized in situ growth and observed by scanning electron microscope image analysis. Noteworthily, 3D hierarchical cobalt sulfides acting as bifunctional photocatalysts are implemented to catalyze the visible‐light‐driven oxygen evolution reaction and hydrogen evolution reaction. This efficient, earth‐abundant, and nonnoble water splitting catalyst for artificial photosynthesis is thoroughly analyzed by various spectroscopic techniques with the aim of investigating its photocatalytic mechanism under visible‐light illumination. The main catalyst of CoS‐2 exhibits considerable H₂ evolution rate of 1196 µmol h^?1 g^?1 and O₂ yield of 63.5%. The efficient activity is attributed to the effective electron transfer between the photosensitizer and catalyst, which is verified by transient absorption experiments. The effective electron transfer between the photosensitizer and catalyst during water oxidation is verified by the dramatic decline of [Ru(bpy)₃]³⁺ concentration in the presence of the catalyst CoS‐2. At the same time, transient absorption experiments support a rapid electron transfers from ³EY* (excited photosensitizer eosin‐Y) to the catalyst CoS‐2 for efficient hydrogen evolution.